The present invention concerns a method and apparatus for electrical discharge or electrochemical machining of workpieces in which the machining electrode is moved relative to a workpiece, the machining comprises cycles and appropriate process parameters are adjusted for machining of the current cycle, and especially with consideration of process parameters of the current cycle.
Numerous cyclic machining methods are described in the prior art, especially planetary EDM methods.
Such a generic planetary EDM method is known, for example, from xe2x80x9cIndustrie-Anzeigerxe2x80x9d, Special Issue, April 1981, page 109ff. This method involves planetary EDM of cavity sinkings, in which a cyclic translatory movement is conducted for machining between the machining electrode and workpiece laterally in planes arranged transversely to the direction of advance. Each plane is then machined cyclically by the machining electrode, the cyclic translatory movements being used in the form of orbital movements to widen a machining or for layered machining.
During electrical discharge machining, material removal occurs on the electrically conducting workpiece, owing to the fact that the machining electrode is brought to the workpiece and electrical discharge occurs. Machining then occurs by the translatory movement between the electrode and the workpiece in an appropriate working liquid. A gap width control superimposed on the translatory movement ensures maintenance of the appropriate spacing between the machining electrode and the workpiece.
It is known from the generic document that the speed of the translatory movement can be controlled as a function of the current process state, in order to avoid losses of efficiency, for example, to carry out the translatory movement with increased speed in the no-load section (sections of the cyclic translatory movement, in which material removal no longer occurs during machining). A shortcoming in this method, however, is that strong fluctuations of the true path can occur within one cycle as a function of the geometry of the reference path in the course of machining. Moreover, braking or delay at tricky sites is problematical at increased speeds. Specifically, it can happen during abrupt changes in direction, for example, that braking is not carried out in a timely fashion and, in the extreme case, the machining electrode can even collide with the workpiece. Because of this, the gap width regulation system can be overtaxed, so that incorrect discharges increasingly occur, which result in increased wear of the machining electrode.
A further developed planetary EDM method is known from EP 0 340 569, which seeks to solve the above problems. Each planetary revolution (cycle)is subdivided into a specific number of angle positions of the planetary angle, and the true path, i.e., the effective deflection amplitude of the machining electrode, is stored for each angle position. The reference path for the pending planetary revolution is then determined in advance from the true values from at least one of the past planetary revolutions and the expected volume of removal. The process runs at almost constant planetary speed. The most erosion-intense discharges possible and therefore a shorter machining time are also sought. The true movement, as mentioned, can be determined from one or more preceding planetary revolutions or, alternatively, from a trial run without erosion. The emphasis in this method therefore lies in accelerating the machining process.
To summarize, it can be stated that this document considers the geometric data of previous planetary revolutions, in order to adjust the geometric data of the pending planetary revolution. Process parameters of previous planetary revolutions are not considered.
In principle, in all known devices, the corresponding CNC control views machining as completed as soon as the undersize is reached. The undersize to be reached, however, refers to a theoretical gap width stored in process parameter data sets. A change in effective gap width, as is usually produced by the gap width control, remains unrecognized, which can have extremely harmful effects on contour accuracy.
The underlying task of the disclosed device is to improve the generic method to the extent that an improved geometric machining accuracy is made possible without time losses, especially to increase process safety and reproducibility.
To solve the task in a generic method, stored process parameters of at least one previous cycle are also used to adjust the process parameters of the current cycle.
The invention proceeds from the observation that, in cyclic machining, a location-bound development of certain process parameters can be established. If a disturbance, for example, occurs at one site of a cycle, the development of this disturbance is also generally apparent in the subsequent cycle at the same site.
Based on this observation, the illustrated device stores the process behavior for already performed cycles and evaluates the stored data to establish certain process parameters of the pending cycle, so that sections of the pending cycle to be machined can be arrived at in advance. For example, depending on whether a disturbance-free section, a no-load section or a critical section is expected, the process parameters to be used are adjusted accordingly. Process disturbances and time losses can therefore largely be avoided to advantage, good shape trueness can be achieved and the process optimized overall.
Preferably, at least one process parameter is measured during at least one preceding cycle. At least one process parameter to be adjusted in the current cycle is derived from this measured parameter. The at least one derived process parameter is stored during the preceding cycle and the at least one stored process parameter is adjusted during the current cycle.
The cycles, including machining (which is also referred to as cyclic machining below), preferably involve planetary erosion during cavity sinking or pocketing during wire erosion. However, any wire erosion machining, in which a full cut and a certain number of aftercuts are carried out, can, in principle, be referred to as cyclic.
Process parameters permanently stored in a table, alternatively or additionally, are preferably used for adjustment of the process parameters, which, in particular, are established with reference to optimization of erosion, with reference to performance of no-load and/or with reference to performance of the guaranteed disturbance-free machining. Stipulated process parameters can therefore also be permanently stored for specific standard situations, like no-load machining, or also for specific compositions of the workpiece, machining electrode, geometry being machined, etc., which can then be called up in specific situations for instantaneous machining. These can be called up, in particular, for those situations when instabilities in process parameter control occur.
The preceding cycles arc preferably investigated by a reference/actual value comparison of the measured process parameter on sections in which a machining disturbance, a no-load condition and/or another condition deviating from the normal case occurs, and adjusted process parameters are set, at least in the corresponding sections of the current cycle. Sections of the cycle are advantageously characterized, in which specific situations occur, and for which optimized process parameters can be worked out in the subsequent cycle with respect to the situation.
Adjusted process parameters are preferably already set in one region before the corresponding sections of the current cycle. For example, abrupt transitions in process parameters that otherwise could lead to machining inaccuracies can therefore be advantageously avoided.
The length of the region is preferably chosen as a function of planetary speed. For the case of wire erosion, the planetary speed corresponds to the wire advance speed.
Machining is preferably divided into a number of layers and sectors and the relevant process parameters are stored layer-wise and sector-wise. For cavity sinking in planetary erosion, the volume being eroded, for example, can be subdivided into layers lying normal to the direction of advance. At the same time, the orbital movement can be divided into sectors, so that, as a result, each machining site can be specified by a sector element. Location-related process information can be allocated and stored in simple fashion on this account.
For wire erosion, these layers can correspond to the full cut and the aftercuts and the sectors are defined along the aftercut.
The process parameters for the current cycle arc preferably set at the beginning of the sector, which are derived from the measured process parameters of preceding cycles, and the process parameters of the current cycle are recorded and stored at the end of a sector as measured quantities for the cycle following the current cycle. Process instabilities are advantageously avoided because of this arrangement. By selecting the sector length, the xe2x80x9ccontrol behaviorxe2x80x9d of the entire process control can then be influenced, i.e., a longer sector length therefore leads to longer sections, in which this process adjustment acts almost as process control.
After adjustment of the process parameters at the beginning of a sector, up to measurement of the process parameters at the end of a sector, only selected process parameters are preferably monitored for particular disturbances. This has the advantage that, during quasi control within a sector, at least those process parameters that can exhibit a significant disturbance situation are monitored. These disturbances can then be reacted to with stored process parameters, which lead in each case to a disturbance-corrected situation.
The stored process parameters after each cycle are preferably multiplied by a weighting factor. This has the advantage that, depending on the situation, certain cycles contribute more strongly or less strongly to determination of the current process parameters. In particular, during corresponding multiplication of all process parameters within a cycle by a factor smaller than one, the process parameters of cycles farther downstream can be included less strongly in an averaging of the process parameters of a specific number of downstream cycles.
With particular preference, cycles with disturbance-affected sections can be multiplied by an increased weighting factor, so that these advantageously have a stronger effect on the subsequent cycles. This disturbance could repeat again, for example, only in the next cycle but one, or in one of the subsequent cycles. By this weighting, special measures can also be taken in these cycles in the relevant sections.
The process parameters to be stored preferably include at least one of the following parameters:
the sum of servo-inteps errors,
the discharge power,
the average discharge current,
the average erosion voltage,
the ignition delay time,
the percentage of short-circuit pulses,
the percentage of instability,
the frequency of program return movements,
the number of interventions of the process to avoid degenerate pulses,
the average pause voltage (i.e., the average voltage during the pause between pulses).
The process parameters to be adjusted preferably include at least one of the following parameters:
the planetary speed
pulse parameters, especially
period duration
pause duration
current
voltage
servo control parameters, especially
amplification
servo reference value
programmed return movements, especially
the frequency of return movements
the stroke of return movements
the strategy during return movements.
The planetary speed is preferably optimized as the process parameter being adjusted with respect to erosion, in which this is determined from several cycles with different planetary speed, each cycle being performed with a constant planetary speed. Advantageously, a planetary speed optimized automatically with respect to erosion can be determined with reference to several test cycles or actual machining cycles.
The planetary speed is preferably optimized as a process parameter being adjusted with respect to machining accuracy, in which a cycle is conducted with a planetary speed reduced relative to the erosion-optimized planetary speed and the planetary speed optimized in machining accuracy is determined by means of the measured process parameters. The machining accuracy can advantageously be strongly increased, especially during the end phase of machining.
The erosion-optimized planetary speed and/or the planetary speed optimized in machining accuracy is then preferably redetermined periodically after a certain number of cycles or on the occurrence of relevant changes in process parameters.